1. Field of the Invention
The present invention relates to a system for providing TDM service, such as DS1, in a GPON system using Virtual Tributary (VT) encapsulation.
2. Background of the Prior Art
A Passive Optical Network (PON) is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 32. A PON consists of an Optical Line Termination (OLT) at the service provider's central office and a number of Optical Network Units (ONUs) near end users. A PON configuration reduces the amount of fiber and central office equipment required compared with point to point architectures. Downstream signals are broadcast to each premises sharing a fiber. Encryption is used to prevent eavesdropping. Upstream signals are combined using a multiple access protocol, typically time division multiple access (TDMA). The OLTs “range” the ONUs in order to provide time slot assignments for upstream communication.
There are a number of standard types of PON that have been implemented. APON (ATM Passive Optical Network) was the first Passive optical network standard. It was used primarily for business applications, and was based on ATM. BPON (Broadband PON) is a standard based on APON. It adds support for WDM, dynamic and higher upstream bandwidth allocation, and survivability. GPON (Gigabit PON) is an evolution of BPON. It supports higher rates, enhanced security, and choice of Layer 2 protocol (ATM, GEM, Ethernet). GPON represents a significant boost in both the total bandwidth and bandwidth efficiency through the use of larger, variable-length packets. A GPON network delivers up to 2,488 megabits per second (Mbit/s) of downstream bandwidth, and 2,488 Mbit/s of upstream bandwidth. GPON Encapsulation Method (GEM) allows very efficient packaging of user traffic, with frame segmentation to allow for higher Quality of Service (QoS) for delay-sensitive traffic such as voice and video communications.
One type of service that is important for business applications is the Digital Signal 1 (DS1) service. DS1 provides twenty-four 8-bit channels, each channel being a 64 kbit/s multiplexed pseudo-circuit, for a total throughput of 1.544 Mbit/s. However, the cost of a GPON system must be optimized for residential service. This includes the cost of the ONU and the allocated share of the OLT. While the business application uses a special ONU, the OLT is common to both business and residential service, thus it is important to provide the capability to serve a significant number of DS1's without embedding significant cost in the OLT. While there will be a future need to provide DS1 service over packet service to the network interface, there will still be an important application to hand off DS1's via SONET encapsulation. Thus, a need arises for a technique by which DS1service can be provided in a GPON system that is cost-effective, flexible, and that is easiy configured and reconfigured.